Morgenstern constructed an explicit arithmetic lattice \(\Gamma \)  in the group \(PSL_2(\mathbb {F}_q((t)))\)  which is isomorphic to the free product \(\langle b_0\rangle *\ldots *\langle b_q\rangle \) , where \(B=\lbrace b_0,\ldots ,b_q\rbrace \)  is a set of elements of order 2 (see [1]}).
The above mentioned Cayley graphs \(\mbox{Cay}(G_i,B_i)\)  are identified as quotients of this \(\Gamma \)  by normal congruence subgroups, where \(B_i = \phi _i(B)\)  is the image of \(B\)  under an epimorphism \(\phi _i\,:\,\Gamma \rightarrow G_i\) .
Note that by [1]} these Cayley graphs are all Ramanujan.
